The non-contact measurement of the surface deformation or displacement in large structures is important when active or passive measures are employed to correct or compensate for the deformation or displacement. For example, a large parabolic surface which is utilized as the reflector of microwave or optical radiation in a microwave or optical telescope is subject to wind and temperature deformations and variations in sag due to dish orientation. To correct for the distortions caused by such deformations either mechanical actuators are used to selectively correct the surface so as to counter the unwanted deformation or mathematical techniques are applied to correct the received signal as reflected by the deformed surface.
Further, such measurements are useful in determining the reaction of many kinds of physical structures to the effects of load even if deformation compensation is not to be applied. For example, the deformation of a building due to wind shear, the deformation of a bridge under traffic load, and the deformation of an airplane wing under flight conditions are all important measurements for the purpose of maintenance and safety.
In each case the amount of surface displacement or deformation which may be very small in magnitude, must be measured precisely, both if the compensation to be applied is to be effective and if the behavior of the structure under load is to be compared with the predicted behavior. Laser range finders may be used in such situations because such range finders permit small displacements to be measured accurately and precisely. However, the cost of such range finders typically requires that one range finder be used to measure the displacement of a surface at multiple points on the surface. In such a case, the displacement or deformation at the various points on the surface is measured sequentially, thereby precluding an accurate measurement of the displacement or deformation of the surface at any point in time.
The present invention provides an inexpensive system for making accurate surface displacement or deformation measurements.